West Nile Virus (WNV) is an important zoo-notic agent having a wide
host range. Due to its emergence with increased virulence in a wide
geographical range, its monitoring becomes imperative. Development of
more rapid and sensitive molecular techniques for instance Reverse
Trans-criptase-Polymerase Chain Reaction (RT-PCR), reverse transcription
loop-mediated isothermal amplification (RT-LAMP) and Nucleic Acid
Sequence Based Amplification (NASBA) assays are vital for detection of
the virus. Various surveillance techniques. according to
epidemiological, climatic and geographical conditions in the exposed
area have also been developed. The surveillance can be set up at
different levels of the WNV transmission cycle using birds, horses and
mosquitoes as sentinels.

West Nile Virus (WNV) is one of the imperative emerging infectious
agents with zoonotic potential. It affects wide varieties of hosts,
produces ranges of clinical manifestations and has emerged with
increased severity with different features and patterns of virulence.
WVN is a positive sense single-stranded RNA enveloped virus of the genus
Flavivirus, family Flaviviridae.

Phylogenetic studies have identified two main lineages of WNV.
Strains from Lineage 1 are present in Africa, India, Australia and the
Western Hemisphere and have been responsible for recent epidemics in
Europe, the Mediterranean basin, the Americas; and strains from Lineage
2 have been reported only in sub-Saharan Africa and have not been
associated with epidemic transmission (1).

The natural cycle of all members of the JE antigenic complex of
Flaviviruses involves birds as the main amplifying host and several
species of mosquitoes as the vectors, ornithophilic mosquitoes,
particularly, Culex species. WNV has been detected in at least 61
species of North American mosquitoes and 328 avian species in United
States (2).

WNV infections acquired through consumption of infected tissues
have been reported for birds, mammals and reptiles. Humans and other
mammals serve as dead-end hosts and do not sufficiently amplify virus
for mosquito transmission, although they may transmit or acquire virus
in utero, through breast milk, via blood transfusion or organ
transplantation, or through occupational exposure (3), (4).

Diagnosis

The definitive detection method for WNV in vertebrate, mosquito
pools and avian samples remain viral isolation which can be performed
from Cerebrospinal Fluid (CSF), blood or tissues in infected cell
cultures. Infected cell culture supernatants or preparations from WNV
infected Suckling Mouse Brains (SMB) are antigens classically used for
WNV serodiagnosis. Alternatively, recombinant antigens such as the
envelope glycol-protein E, Virus Like Particles (VLP), or the
nonstructural NS 1, NS3 and NS5 proteins may be used in different assay
formats in the absence of particular containment facilities (5).

Antibody testing in human or animal sera is of large usefulness for
diagnosing WNV infection but cross-reactivity limits diagnostic
specificity. WNV specific IgM and IgG capture ELISA tests and the Plaque
Reduction Neutralization Test (PRNT) detecting WNV specific neutralizing
antibodies in CSF and serum remains the assay required for confirmation
of flavivirus infections. MAC-ELISA test is the most efficient for
detection of IgM antibody and is valuable for serosur-veillance. The new
microsphere immunoassay provides a sensitive and rapid alternative to
traditional ELISA that detects antibodies to flavivirus E proteins (6).

The presence of the virus can be confirmed by nucleic acid
detection. A sensitive and WNV-specific reverse transcription and nested
PCR method has been used successfully (7). In addition to greatly
enhancing detection sensitivity, the shorter turn-around time of
real-time PCR has made it a more favoured diagnostic technique. A
sensitive real-time PCR assay. incorporating Fluorescence Resonance
Energy Transfer (FRET) probes have been designed for the rapid and
simultaneous detection and genotyping of WNV (8).

Immunohistochemistry on CNS tissues using WN-specific MAb and
Antigen capture ELISA tests w confirm the presence of WN V in avian
tissues and mosquito pools are very imperative diagnostic methods.
Advanced nucleic acid based techniques like TaqMan reverse
transcriptase-PCR assay and Nucleic Acid Sequence Based Amplification
(NASI3A) assays have demonstrated a greater sensitivity than the
traditional RT-PCR method (9). A one step, single tube Real-time reverse
transcription loop-mediated isothermal amplification (RT-LAMP) assay is
another novel method of gene amplification developed for rapid detection
of the envelope gene of WNV (10)

Surveillance

The surveillance of WNV can be performed by using passive
surveillance system or by active surveillance depending on
epidemiological, climatic and geographical conditions in the exposed
area. Most of the threatened countries have organized a passive
surveillance with horses, birds and humans. Horses serve as good
sentinels for WNV infection surveillance because they are easily
identifiable; their role in the epidemiological cycle of WNV; being
dead-end hosts like humans; low cost easy to maintain facility to
capture and sample horses: and the availability of serological tools are
to do surveillance.

Birds have always been considered useful candidates as sentinels
for the presence of this virus in a geographical area. Mosquito testing
is not a practical method for routine surveillance of transmission,
because the proportion of WNV positive mosquito pools in wild
populations is very low even when transmission rates are high.

In conclusion, emergence of WNV as a major cause of public health
concern and development of advance molecular techniques for the
diagnosis and surveillance has enhanced our understanding of the pattern
of development and spread of WNV in different geographical regions of
the world.